Lithium-ion secondary batteries with high energy densities have been used as power sources for portable electronics such as a mobile phone and a notebook computer.
An electrode member of a lithium-ion secondary battery includes a positive electrode material, a separator, and a negative electrode material. Regarding a positive electrode material, an aluminum alloy foil, having excellent electrical conductivity and less heat generation without affecting electrical efficiency of a secondary battery, has been used. Here, aluminum alloy of JIS1085 and JIS3003 have been generally used. The positive electrode material can be obtained by applying an active material having a lithium-containing metal oxide such as LiCoO2 as a chief component on both sides of the surface of the aluminum alloy foil, followed by drying, and then subjecting to compression forming using a press machine (this step is hereinafter referred to as press working). The positive electrode material as so prepared, a separator, and a negative electrode are stacked, and then the resulting stack is wound. After a shaping process is performed, it is encased.
An aluminum alloy foil used for a positive electrode material of a lithium-ion secondary battery has several problems that cuts occur during application of an active material and that ruptures occur at a bending portion during winding. Thus, a higher strength is required. At a drying step after the application of the active material, heat treatment is carried out at about 100 to 160° C. Accordingly, such heat treatment lowers the strength of the aluminum, and thus it is likely to generate middle waviness during press working. This induces wrinkles during winding, which reduces adhesion between the active material and the aluminum alloy foil. Besides, a rupture is likely to occur during a later slitting process. In particular, when the adhesion between the active material and a surface of the aluminum alloy foil decreases, their peeling is facilitated during repeated operation of discharge and charge. Unfortunately, this causes battery capacity to decrease.
Recently, a high electrical conductivity has been required for an aluminum alloy foil used for a positive electrode material of a lithium-ion secondary battery. What is meant by the electrical conductivity is a physical property indicating how easily electricity is conducted in a substance. The higher the electrical conductivity is, the more easily the electricity is conducted. Lithium-ion secondary batteries used for automobiles and/or electric tools necessitate a higher output characteristic than lithium-ion secondary batteries used for consumer-use mobile phones and/or laptop computers. When a large current flows, a lower electrical conductivity causes increase in the internal resistance of a battery. Consequently, this reduces output voltage of the battery.
An aluminum alloy foil with the Al purity of 99% or more has been used as an alloy foil for a lithium-ion secondary battery, which requires a high electrical conductivity. However, the aluminum alloy foil with the Al purity of 99% or more makes it difficult to improve its strength because the foil contains a fewer amounts of elements which contribute to the improvement in strength. That is, since there are fewer solid-solution elements or fine precipitates that can suppress the dislocation movement during heat treatment, the strength is largely decreased by the heat treatment conducted when the active material is applied.
For example, when the JIS 1000 series aluminum alloy is cast by semi-continuous casting of the molten metal alloy, a specific cast structure referred to as the “feather-like structure” is frequently found in the ingot thus obtained. This feather-like structure is a thin sheet-like growth twin, and is a cause for cracks during casting or sheet cracks during rolling. It has been known that generation of the feather-like structure can be suppressed by the addition of a refiner. However, when the refiner is added by a large amount, the solid solution content in the aluminum matrix would increase, which is considered to decrease the electrical conductivity at a large extent.
Therefore, concerning an aluminum alloy foil with high purity, it is extremely difficult to suppress the generation of the feather-like structure and to prevent the sheet crack during rolling, while maintaining high conductivity.
That is, as for materials for electrode current collectors, in particular, as for electrode materials for lithium-ion secondary batteries, an aluminum alloy foil having a high strength after the heat treatment during the drying step after the preparation of the aluminum alloy foil for electrode current collectors, and having superior foil rolling property for the improvement in productivity while maintaining the high electrical conductivity, has been desired.
Patent Literature 1 discloses an aluminum alloy foil with a tensile strength of 98 MPa or more, which is used for a battery collector. Patent Literature 1, however, is silent on its strength after a drying step in a manufacturing process of a positive electrode material for a lithium-ion secondary battery.
Patent Literature 2 discloses an aluminum alloy foil with a tensile strength of 160 MPa or more, which foil is used for an electrode current collector of a lithium-ion secondary battery. However, the strength after heat treatment, which simulates a drying step, is low. This strength is insufficient for preventing wrinkles during winding and ruptures during a slitting process because middle waviness occurs during press working.
Patent Literature 3 sets forth a method for preventing peeling from an active material without inducing plastic deformation during press working by increasing the strength. However, the alloy used contains Mn, Cu, and Mg as principal elements. Therefore, it is impossible to achieve a high electrical conductivity.